Sen. Ron Johnson of Wisconsin speaks with reporters in Green Bay in May.

Scott Bauer / AP

In recent elections, more and more voters have been choosing candidates from the same party for president and Senate. That trend appeared to be holding true this year too, even with Donald Trump, unusual as he is, on the ballot. So as Hillary Clinton jumped out to a bigger lead in the polls starting after the first presidential debate in late September, we might have expected Democratic Senate candidates to poll better as well. That hasn’t happened — the chance of Democrats controlling the Senate is only 54 percent in our polls-only model and 56 percent in our polls-plus model.

Indeed, the races for Senate control and the White House have split. Here are the Democrats’ chances of taking back the Senate and winning the presidency, according to our polls-only forecasts, from mid-July until now.1

Clinton’s pre-Democratic-convention swoon was matched by Democrats doing worse in the polls. But Democrats rebounded as Clinton did after the convention. Then their odds all fell together in the second half of August and through September. Clinton’s chances began to rise again after the first debate, but unlike after the convention, Democrats’ chances of taking back the Senate haven’t followed Clinton’s presidential odds upward. To put this in mathematical terms, the correlation between the Democratic chances in the Senate and Clinton’s chances was a very high +0.87 in the 74 days before the first debate.2 In the 16 days since, it’s been -.23, indicating that they’re moving in opposite directions.

This negative relationship between Clinton and Democratic Senate candidates seems to hold true across the board. We can see this by looking at the FiveThirtyEight polls-only forecast on the date of the debate and on Oct. 12 for both the Senate and the presidency in the 26 states where at least one poll was conducted before the first debate.

SEPT. 26

OCT. 12

CHANGE

STATE

PRESIDENT

SENATE

PRESIDENT

SENATE

PRESIDENT

SENATE

Arizona

-4.9

-11.7

+0.5

-13.6

+5.4

-1.9

Arkansas

-18.7

-14.1

-15.1

-18.8

+3.5

-4.6

Colorado

+1.6

+9.3

+7.1

+10.1

+5.6

+0.8

Connecticut

+10.1

+23.1

+17.1

+22.3

+7.0

-0.7

Florida

-1.0

-5.3

+4.1

-5.9

+5.0

-0.6

Georgia

-6.1

-14.7

-2.5

-15.7

+3.6

-0.9

Idaho

-23.6

-33.4

-18.4

-33.4

+5.2

0.0

Illinois

+10.9

+4.0

+17.1

+7.7

+6.2

+3.6

Indiana

-12.4

+11.9

-6.6

+7.4

+5.8

-4.5

Iowa

-3.7

-11.2

+1.6

-13.3

+5.3

-2.1

Kansas

-12.5

-20.1

-8.3

-21.0

+4.2

-0.9

Kentucky

-18.3

-10.0

-12.6

-10.5

+5.7

-0.5

Louisiana

-15.5

-26.2

-12.0

-26.6

+3.5

-0.4

Maryland

+22.6

+29.8

+29.3

+29.3

+6.6

-0.5

Missouri

-9.2

-2.9

-4.9

-3.0

+4.3

-0.1

Nevada

-1.0

-2.5

+4.7

-3.3

+5.6

-0.7

New Hampshire

+2.3

-0.5

+8.3

-1.8

+6.0

-1.3

New York

+16.8

+43.0

+21.3

+42.1

+4.6

-0.9

North Carolina

-1.6

-1.2

+3.2

-1.6

+4.7

-0.4

Ohio

-2.0

-11.3

+2.7

-13.5

+4.7

-2.1

Pennsylvania

+2.4

+2.0

+7.5

+0.4

+5.1

-1.6

South Carolina

-10.2

-25.9

-5.9

-26.8

+4.4

-0.9

Utah

-17.0

-28.4

-12.1

-31.2

+5.0

-2.9

Vermont

+21.3

+32.4

+27.9

+32.0

+6.7

-0.4

Washington

+9.9

+21.3

+15.3

+12.3

+5.4

-9.0

Wisconsin

+3.5

+9.5

+8.3

+4.9

+4.8

-4.6

Average

+5.1

-1.5

Unlike Clinton, Democratic Senate candidates are not gaining

Races where at least one Senate poll was conducted before Sept. 26

Source: Polls-only forecast model

Clinton has improved her margin over Trump by an average of 5.1 percentage points in these 26 states, while the average Democratic candidate’s margin is down 1.5 percentage points. And while Clinton has improved her margin by at least 3.5 percentage points in every one of these 26 states, Republican Senate candidates have improved their chances in all but four: The Democratic candidates have gained ground in Colorado and Illinois, and the races in Idaho and Missouri have stayed roughly unchanged.

So what the heck is going on? The most hopeful hypothesis for Democrats is that there’s a lag. That is, eventually these Senate races will start reflecting the presidential race. That’s certainly plausible, given voters’ recent pattern of picking the same party in both Senate and presidential races. Down-ballot races can break late, as they did in 2006 when Democrats did considerably better than expected based on predictions from a month before the election.

Another, far less optimistic hypothesis for Democrats is that voters are purposely splitting their tickets. As my colleague Nate Silver pointed out on Tuesday, there’s some evidence that voters split their tickets when they feel confident in predicting who the next president will be. If they’re certain it will be a Democrat, they’ll vote for a Republican for Senate, and vice versa. It’s known in political science as “anticipatory balancing.” With Clinton’s lead becoming clearer by the day even as her favorability rating remains low (albeit not as low as Trump’s), it wouldn’t be surprising to see voters seeking a Republican Congress as a check on a President Clinton.

Obviously, we’ll know more in the weeks ahead. All we can say for sure right now is that the race for the Senate remains very tight. Democrats have the slightest of advantages in the race for a majority, but it’s basically a toss-up. As I noted last week, there’s still a wide range of possible outcomes. We may not know with any real certainty who will take control of the Senate until the votes are actually counted.